Might need to check with local codes or local recommendations. I do know that all vents should be significantly higher than snow depth that is a normal high for your area.... in Alabama a typical vent is about 12" above the roofing.

The closer to the roof, the better. Roof caps are available that have backdraft dampers in them. Most fans have them as well. This will prevent the outside air from getting in.
With fans, you get what you pay for. A $20-$30 fan will be noisy, get worse and do a poor job.
Install something around the 110CFM area, with a 4" duct. Use ridgid duct, tape all the seams and insulate the duct, sealing everything up tight. vapor barrier over the fan to bring it into the building envelope.
The short distance and the insulated duct will allow some air flow out of the house, past the backdraft dampers. These are both gravity controlled dampers so there isn't much of a seal. This minor air flow keeps the snow away from the opening. When the fan is on, you will notice a large swath of bare roof directly below it.

I know my local Home Depot has a display of fans. Each one is labeled in CFM (cubice feet per minute) of air flow as well as decibel (noise) level. Plus you can actually turn each on to get a listen of the actual noise level. You can really listen to how much quieter the more expensive squirel cage fans are. I spent about $Cdn 150 for one I put into my kids bathroom reno and it is very quiet.

Most people don't leave the fan on long enough. Leaving it on too long wastes energy. The ideal way to control one is to use a hygrometer controlled switch - i.e., one that monitors the relative humidity, and turns itself off (and on) when needed.

I agree with Jim that a timer or a hygrometer-triggered shutoff is the only way to go, in the long term. Nobody wants to have to "go to the bathroom" twice, the second time just to handle the air system. By this, I mean turn it off.

Molo, in terms of your priorities, i think you will want to focus on how your building handles air in broadest sense of the term "handle". Every time you create a new opening to the outdoors you are allowing convection (warmer air rises) to push your indoor air to the outside, all the time, even when the fan is off. Replacement air comes from somewhere else in the house, although rarely you can have a slow return current in the vent conduit itself. Inside a (3", 4", or larger, bathroom kitchen or dryer) vent there is usually a single direction of flow; it may seem very slow to you on a "normal" day but it is always ON by the fact that it is always OPEN, and it loses a lot of air to the outside. Furthermore, the air flow can be extremely fast and powerful without any motors pushing air. Pressure differentials are normal in nature, and that's what makes wind blow, and that's what sucks air out of your house on one side and pushes it in somewhere else.

The other big air openings in the house are probably in the kitchen (over the cooktop), and for the fireplace / wood stove / heating system. In my place, sometimes (rarely) air comes down one of these vents when it is needed to fill the void created by another vent... so it really is a good idea to plan for a heat exchanger / air exchanger system, as they equalize pressure automatically. It is pretty ugly when a fireplace not in use lets air come down the chimney spreading the odor of soot and ashes throughout the house -- this happens every year, in my house. Yes, there is a damper that closes the flue, and even if I seal the fireplace off with tape, there is still a lot of air pressure pushing air inside... Could be because of how wind blows around the vents above, or the fact that the chimney area is the coldest corner and that lets convection push air up out of the other vents first, causing a negative pressure.... The point to remember is that you might want to be able to close the vent entirely at certain times, and that whole-house air/heat exchangers are good to have in very cold climates.

Don't worry too much about eliminating moisture. yes, it is important to exchange air in the bathroom, to get rid of the moisture, but no, it is not the single most important thing to focus on, in a cold climate where you need to conserve heat and plan for makeup air to come in when a fan is pushing air out. Handling air exchanges, air flow, airtightness, and insulation, is just as important.

Water vapor is invisible at most concentrations, and it is the lightest gas in the air so it rises fast. To repeat this: O2, N2, CO2 are all heavier than H20 in gaseous form. This explains why wator (vapor, humidity, moisture, condensation, etc) is a problem upstairs or in attics when a house has too much humidity. You cannot get rid of humidity, since you breathe all the time and this puts moist air into your house. I bought a home hygrometer and it taught me a lot. One thing being not to worry about the general humidity level in the winter.

Is your building leaky or airtight? Modern or old? Do you have a whole house air handling system, or do you rely on air leaking and doors opening to exchange air? How well insulated is the house?

Quote: Geniescience
"Don't worry too much about eliminating moisture. yes, it is important to exchange air in the bathroom, to get rid of the moisture, but no, it is not the single most important thing to focus on, in a cold climate where you need to conserve heat and plan for makeup air to come in when a fan is pushing air out. Handling air exchanges, air flow, airtightness, and insulation, is just as important."

The moisture meter switch sounds great! I thought that moisture elimination was very important. Especially in a bathroom with no window. Heat loss is a concern too.
This is a small 2-bedroom home with no range vent, no fireplace openings, and no air handling system. The bathroom vent would be the only opening to the outside (with the exception of windows and one door).
So what's the verdict? moisture removal less important than heat loss?

... no range vent, no fireplace openings, and no air handling system. The bathroom vent would be the only opening to the outside (with the exception of windows and one door).

Click to expand...

molo,

You need a fan, no doubt about it, and that requires a vent opening to the outside, too, no doubt about it.

What would be great but so far I have never seen it, is to have a fan that closes air flow to avoid losing nice warm air from your conditioned space, and then both opens the vent pipe and blows air out when it gets triggered by one of a few events (E.g. starting a minute after someone turns on the light until five minutes after they turn the light off, or when someone flushes the toilet or turns on the shower). Then you'd have both the trigger and a partially closed air passage the rest of the time.

I bought round 4" vent blocker plates that have a pivot (you drill two small holes on opposite sides of the vent pipe, you stick a wire through and use it to pivot the round plate on, so you can close it and open it manually. Even then, a lot of air still leaves the house when the vent plate is turned to the closed position. It is still very leaky even though almost 99% of the visible opening is covered. Air pressure differences cause a lot of air to move through very small spaces. Warm air really wants to leave your house and go outside.

So the best tip i can give you is to get a vent that is real quiet and that you can close manually so that you block the air 99%. In one of my vents I have stuffed a dozen plastic bags into it when I wanted to block air during big winter wind storms. The temperature difference between indoor and outdoor air can be 100 degrees and when your vent pipe is going straight UP, the amount of air you lose is huge !

I forgot to mention dryer vents, and you didn't mention a dryer vent either... You may want to be able to shut that vent too.

the right question to ask ain't "So what's the verdict? moisture removal less important than heat loss?" because they are both important, and I wanted to be sure to communicate that other things were also important, instead of overfocusing on one thing. A lot of people have made a lot of bad judgement calls because they wanted too much to do the right thing or to build good things into their project -- which caused them to overlook other important things. You wouldn't have a tendency to do that, would you?

On 01-03-2007, at 3:19PM, you wrote, "I want a very powerful vent that doesn't leave any moisture in this bathroom. Any ideas how to tell how powerful a vent is?"

It's a small house, in a cold climate. What you REALLY want in a small bathroom in a small house is not a big fan. You want a fan with a small motor, with easy access to the vent opening so you can manually close the vent whenever you might want to. The smaller the fan, the more likely it is that its design will be quiet by nature, and the manufacturer's claim about noise (or low-noise) will be accurate. The quieter it is, the more people enjoy leaving it on.

When you buy a fan, its capacity is written on the box. That is how you tell how powerful a vent is.

The cost can vary by a factor of ten. $20 to $200. In one bathrooom I spent $200 for a remote squirrel cage and a closeable vent opening, and $100 for a small ultraquiet (but not squirrel cage) fan in the other bathroom. I know I could have spent less than that, but I really wanted peace and quiet, not the whirring noise of "functional" motors that "do the job".

Dampers don't necessarily seal all that well. I used a tubular skylight made by Solatube with their vent/fan kit. I used their in-line damper and ran it through a vent cap on the roof with one of its own. I don't notice cold air, even when it gets windy, nor do I hear the flaps banging. If you want some more light in the room, you might want to consider this option - the fan is remote, quiet and it lets in a bunch of free light (they claim the equivalent of 300W light bulb) in full sun. www.solatube.com